This invention relates generally to ebulliometry. More particularly, this invention relates to an ebulliometric technique for detecting localized hot spots on small objects, such as integrated circuits.
Operating microelectronic devices may have highly nonuniform temperature distributions resulting mainly from ohmic heating. The nonuniform distributions may result in localized hot spots which induce early failures of the devices. On printed circuit boards, for example, localized heating may result from defects in metallizing, such as excessive metal etching or poor plate-through connections, which in turn cause the current carrying area to be insufficient under load. Poor heat sink design and inadequate solder connections may also result in localized heating of printed circuit boards. Localized heating causes hot spots and related failure problems. Integrated circuits are subject to troubles similar to those which may afflict printed circuit boards.
Detection of hot spots, particularly in integrated circuits, becomes increasingly difficult as the size scales of the circuits decrease.
Prior art techniques for detecting hot spots are discussed in Blackburn, "An Electrical Technique for the Measurement of the Peak Junction Temperature of Power Transistors," Proc. IEEE 13th Annual Conf. on Reliability Physics (1975), and Brenner, "A Technique for Measuring the Surface Temperature of Transistors by Means of Fluorescent Phosphor," National Bureau of Standards Technical Note 591 (1971).
One prior art method of locating hot spots is the scanning infrared microscope. This device detects the infrared radiation emitted from electronic circuits under test. Infrared detection systems are complex, expensive, and difficult to use. Further, they are not suitable for use with large scale integrated circuits because of the inherent resolving power limitations of infrared systems. An infrared detection system can only resolve emission regions having sizes of the order of the wave length of the detected radiation, or greater. For example, a modern high quality infrared microscope, discussed in Barnes Engineering Co. Bulletin 12-910C, having a spectral response of about 1.8 to 5.5 microns can resolve emitting regions greater than about 7.6 microns diameter. Present-day semiconductor devices very often have structures which are less than one micron in size; these structures cannot be resolved under an infrared microscope.
Infrared detection of hot spots is also not conducive to the determination of absolute hot spot temperatures because of the dependence of the infrared radiation upon the emissivity of the radiating region. A semiconductor device has a number of materials with different emissivities such as bare silicon, silicon oxide, metallized conductors, and passivating glass. Present practice, therefore, is to coat a device with black lacquer in order to obtain uniform emissivity. The coating, however, obscures the view of the precise structure of the device under test. Also, the coating, being a good radiator, alters the thermal characteristics of the device, thereby making quantitative interpretation of measurements difficult. When coatings are not used, metallized conductors may be particularly troublesome because the conductors act as mirrors and give false indications of hot spots by generating reflections from nearby light sources.
Brenner describes a technique for the determination of hot spot temperatures by coating a transistor chip with a temperature-sensitive phosphor. The phosphor's fluorescence when irradiated by ultraviolet light of suitable intensity is strongly temperature-dependent. Brenner mounted the coated chips on a temperature-controlled heat sink and applied various voltages to the chip. The fluorescent pattern was photographed through an enlarging lens and the film analyzed with a microdensitometer to extract temperature data.
Brenner's technique is not useable for devices having connecting wires obscuring the surfaces, heavy metallization, or external coatings or packaging. The resolution is limited by the phosphor grain size which can occur in clumps up to 50 microns in diameter. The phosphor coating technique is therefore unsuitable for hot spot detection on integrated circuits having structures with sizes of the order of a micron or less.
Another known technique described in "Determining Hot Spots, Temperatures on IC Chips," Circuits Manufacturing (September 1976), involves placing a device on a thermoelectric element which is cooled to a temperature below the condensation temperature of nitrogen gas saturated with a fluorocarbon liquid. The saturated gas is blown across the device until the fluorocarbon forms a condensation layer on the device surface. Power is then applied to the device until hot spots exhibit themselves as breaks in the condensation layer. The surface temperature of a hot spot at any given power input is determined by cooling the thermoelectric element to a second temperature at which condensation on the hot spot recurs. It is assumed that the difference between the temperatures of the hot spot and thermoelectric device is essentially independent of the thermoelectric device temperature. The hot spot temperature at any ambient temperature is therefore assumed to be the temperature difference between the ambient and second thermoelectric element temperatures plus the condensation temperature of the fluorocarbon liquid. Observations of the breaks in the condensation layer and the subsequent recondensation are made through a microscope.
One difficulty in the condensation technique is that it does not reveal hot spots which are not directly in the field of view of the microscope. Thus, it may be necessary to observe a device from many aspects in order to obtain complete coverage for hot spot detection. Another difficulty is that tests are limited to device temperatures in the range of about 0.degree. C. to 70.degree. C. It would be desirable to be able to test over a much greater temperature range. Finally, hot spot resolution is limited by surface tension effects and is probably not better than several microns.
More recent work, some of which is described by G. P. Dunden, Semiconductor International '80, Brighton, England (1980), involves immersion of a device in a fluorocarbon liquid. Hot spots manifest themselves as boiling sites. The technique has been used to detect hot spots on printed circuit, multi-layered and wire wrap boards. The technique has not previously given good results for integrated circuits because the bubbles emitted from hot spots disturb the fluid surface, making it difficult to see individual boiling sites and otherwise obscuring the view of the boiling sites. The technique is in any event limited to the detection of hot spots having temperatures above the boiling point of the particular liquid used. The boiling technique has not been applied to hot spot temperature determination.